The present invention relates generally to electric propulsion systems for traction vehicles, and it relates more particularly to improved means for controlling such a system during electric braking of the vehicle.
An electric propulsion system for a traction vehicle, such as a large haulage truck, typically comprises a prime mover-driven electric generating means for supplying electric power to high horsepower traction motors respectively connected in driving relationship to a pair of wheels on opposite sides of the vehicle. The prime mover is commonly a diesel engine, and the traction motors are generally adjustable speed, reversible direct current (d-c) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector lever. This speed control pedal is adapted to control the engine speed (rpm) which determines the power output of the generating means, thus varying the magnitude of the voltage applied to the traction motors.
Deceleration of a moving vehicle is accomplished by releasing the speed control pedal and either allowing the vehicle to coast or activating its mechanical or electrical braking system. In the electric braking mode of operation the motors behave as generators, and the magnitude of the voltage generated across the armature windings of each motor is proportional to the rotational speed and the field excitation current of the motor. Dynamic braking resistor grids are connected across the armatures of the respective motors to dissipate the electric power output of the motors during electric braking. The average magnitude of current in each resistor grid is a measure of the braking effort of the associated motor.
The magnitude of motor field excitation current (and hence the braking effort) of each motor during electric braking is a function of the value of a braking call signal. In prior art systems the braking call signal value is determined by the position of a retard foot pedal that the vehicle operator can manually manipulate. To initiate braking, the operator first releases the aforesaid speed control pedal and then steps on the retard foot pedal. If the vehicle is accelerating down a hill, the operator ordinarily will allow it to coast until the vehicle speed approaches a maximum safe limit, whereupon the retard foot pedal is depressed so as to initiate electric braking and increase the braking call signal. By appropriately varying the position of the latter pedal, the maximum safe speed can be approximately maintained. This in effect is an open loop form of retard speed control. A high degree of skill is required to modulate the foot pedal position so as to maintain the vehicle speed close to optimum when going down hill on a slippery surface. If and when the operator senses that the vehicle is gaining speed while the retard pedal is fully depressed, the mechanical braking system of the vehicle must be activated.
It is common practice to protect the vehicle from overspeed by providing suitable means for automatically switching from the motoring mode of operation to an electric braking mode of operation in response to the vehicle speed exceeding a predetermined high level and for subsequently restoring the motoring mode when the vehicle slows down to a safe speed. In prior art systems the overspeed responsive means sets the braking call signal at a maximum value, whereby the full amount of braking effort is obtained. If the operator were to rely on overspeed protection to control the speed of an unloaded vehicle while traveling down a long hill, the powered wheels of the vehicle would be subjected repeatedly to high mechanical stress and their fatigue life could be undesirably shortened.
If the surface of the road bed on which the vehicle is traveling is wet or icy, there is a possibility that one or both of the powered wheels of the vehicles may start to skid or slide during electric braking. In this event it is desirable to reduce the value of the braking call signal in order to allow the sliding wheel to resume rotating at vehicle speed.